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United States Patent |
5,732,167
|
Ishiko
,   et al.
|
March 24, 1998
|
Optical fiber sensor for measuring a magnetic field or electric current
and method for making the same
Abstract
An optical fiber sensor which is adapted for use in combination with a
horseshoe-type iron core comprises a substrate having a groove pattern
established by a pair of elongated spaced, parallel grooves, and a third
groove which intersects the paired grooves at right angles. An optical
fiber having a U-shaped portion is inserted into and fixed in the groove
pattern, and an optical modulation unit, which is fixedly provided in a
light path of the optical fiber, is placed in the third groove. The
substrate is a composite which comprises a non-magnetic portion and soft
magnetic portions fixed to either side of the non-magnetic portion. The
paired elongated grooves are formed in the soft magnetic portions, while
the optical modulation unit is fixed to the non-magnetic portion. A method
for making such a composite substrate is also described.
Inventors:
|
Ishiko; Daisuke (Osaka, JP);
Minemoto; Hisashi (Ootsu, JP);
Itoh; Nobuki (Osaka, JP)
|
Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
Appl. No.:
|
627315 |
Filed:
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April 3, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
385/12; 324/244.1; 385/1; 385/4; 385/6; 385/9 |
Intern'l Class: |
G02B 006/00 |
Field of Search: |
385/12,4,6,34,1,11
324/244,244.1,260,96
250/225,227.17
|
References Cited
U.S. Patent Documents
4962990 | Oct., 1990 | Matsuzawa et al. | 385/34.
|
5202629 | Apr., 1993 | Seike et al. | 324/244.
|
Foreign Patent Documents |
63-60410 | Mar., 1988 | JP.
| |
1308970 | Dec., 1989 | JP.
| |
363606 | Mar., 1991 | JP.
| |
3170071 | Jul., 1991 | JP.
| |
5297027 | Nov., 1993 | JP.
| |
Other References
"Fiber -Optic Monitoring Sensor System for Power Distribution Lines" by D.
Ishiko et al, National Technical Report vol. 38, No. 2, pp. 255-261 (1992)
Apr.
"Submicroampere-per-root-hertz current sensor based on the Faraday effect
in Ga:YIG" by A.H. Rose, et al, Optics Letters Vo. 18, No. 17, pp.
1471-1473 (1993) Apr.
"Polarization-Independent In-Line Optical Insolator with Lens-Free
Configuration" by Shiraishi et al, Journal of Lightwave Technology, vol.
10, No. 12, pp. 1839-1842 (1992) No Month.
|
Primary Examiner: Palmer; Phan T. H.
Attorney, Agent or Firm: Lowe, Price, LeBlanc & Becker
Claims
What is claimed is:
1. An optical fiber sensor which comprises:
a substrate having a pair of elongated grooves kept away from and in
parallel with each other and formed along the length of the substrate, and
a third groove intersected at right angles with the paired grooves,
respectively;
an optical fiber having a U-shaped portion which is inserted into and fixed
in a groove pattern formed by said paired grooves and said third groove;
and
an optical modulation unit which is fixedly provided in a light path of
said optical fiber placed in the third groove, wherein the substrate is
composed of a composite substrate which comprises a non-magnetic portion
and soft magnetic portions fixed to the non-magnetic portion at opposite
sides thereof, said paired elongated grooves are, respectively, formed in
said soft magnetic portions, and said optical modulation unit is fixed to
said non-magnetic portion.
2. An optical fiber sensor according to claim 1, wherein said optical
modulation unit includes a polarizer, a magnetooptical element and an
analyzer sequentially arranged in this order in the optical path wherein
said polarizer, said magnetooptical element and said analyzer are,
respectively, disposed and fixed in grooves formed in said non-magnetic
portion at intervals therebetween.
3. An optical fiber sensor according to claim 2, wherein said polarizer,
said magnetooptical element and said analyzer are, respectively, designed
to be higher than the depth of the respective grooves.
4. An optical fiber sensor according to claim 2, wherein said
magnetooptical element is disposed and fixed in a groove formed in said
non-magnetic portion and said polarizer and said analyzer are,
respectively, disposed and fixed in grooves which are formed in said soft
magnetic portions, respectively.
5. An optical fiber sensor according to claim 2, wherein said polarizer,
said magnetooptical element and said analyzer are fixedly bonded together
to provide an integrally bonded optical modulation unit in the optical
path whereby said non-magnetic portion is made narrow sufficient to
receive said integrally bonded optical modulation unit.
6. An optical fiber sensor according to claim 1, further comprising another
composite substrate which has a non-magnetic portion and soft magnetic
portions fixed to said non-magnetic portion at opposite sides thereof and
which has a similar construction as the first mentioned composite
substrate wherein said another composite substrate is fixed to said
first-mentioned composite substrate so that the soft magnetic portions are
facing each other.
7. An optical fiber sensor according to claim 6, wherein said soft magnetic
portions of said another composite substrate are higher than said
non-magnetic portion whereby said another composite substrate is recessed
at said non-magnetic portion.
8. An optical fiber sensor according to claim 1, wherein said soft magnetic
portions, respectively, have a thickness equal to said non-magnetic
portion.
9. An optical fiber sensor according to claim 1, wherein said soft magnetic
portions, respectively, have a thickness greater than said non-magnetic
portion.
10. An optical fiber sensor according to claim 1, wherein said non-magnetic
portion consists of glass or a ceramic, and said soft magnetic portion
consists of a ferrite.
11. An optical fiber sensor according to claim 1, wherein said U-shaped
portion have two bent portions which have, respectively a radius of
curvature of from 0.3 to 5 mm.
12. An optical fiber sensor according to claim 1, further comprising a pair
of soft magnetic pieces mounted on and fixed to said soft magnetic
portions, respectively.
13. A method for making an optical fiber sensor comprising the steps of:
forming a substrate having a pair of elongated grooves kept away from and
in parallel to each other and formed along the length of the substrate,
and a third groove intersected at right angles with the paired grooves,
respectively;
inserting an optical fiber having a U-shaped portion into and fixing the
optical fiber in a groove pattern formed by said paired grooves and said
third groove; and
inserting an optical modulation unit in the third groove so as to be in a
light path of said optical fiber and fixing the optical modulation unit in
the third groove;
wherein said substrate is formed by bonding soft magnetic portions to a
non-magnetic portion at opposite sides thereof by means of a bonding
agent, thereby forming a composite substrate.
14. A method according to claim 13, further comprising the step of forming
the pair of elongated grooves in said soft magnetic portions,
respectively, after the bonding.
15. A method according to claim 13, wherein said bonding agent consists of
an organic bonding agent.
16. A method according to claim 13, wherein said bonding agent consists of
an inorganic agent and is fired after bonding.
17. An optical fiber sensor comprising:
a composite substrate having a non-magnetic portion and a magnetic portion
attached to a side of the non-magnetic portion;
an optical modulation unit having an optical element attached to said
composite substrate; and
an optical fiber attached to said composite substrate so as to direct light
along a path towards said optical modulation unit wherein:
said composite substrate includes a first groove and a second groove; and
said optical fiber is disposed in said first groove and said optical
element is disposed in said second groove.
18. An optical fiber sensor according to claim 17, wherein the modulation
unit is mirrorless, and the modulation unit and optical fiber are fixedly
attached to said composite substrate.
19. An optical fiber sensor according to claim 17, wherein said first
groove is substantially perpendicular to said second groove.
20. An optical fiber sensor according to claim 17, wherein:
said magnetic portion is a first soft magnetic portion; and
said composite substrate includes a second soft magnetic portion attached
to an opposed side of said the non-magnetic portion.
21. An optical fiber sensor according to claim 17, wherein said optical
fiber includes a curved portion.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical fiber sensor which is adapted for the
measurement of a magnetic field or electric current by using light beams.
The invention also relates to a method for making an optical fiber sensor
of the type mentioned above.
2. Description of the Prior Art
As optical element-making techniques, optical measuring techniques and
optical communication techniques have been recently advanced, optical
sensors and miniaturized optical devices for optical communication systems
have been put into practice in various fields, or are under study or
investigation for practical usage. For instance, optical fiber sensors
have not only good insulating properties, but also good characteristics
such as a high resistance to electromagnetic inductive noises resulting,
for example, from thunder. Accordingly, such sensors are being reduced to
practice as optical fiber current or voltage sensors in the field of the
electric power (National Technical Report Vol. 38, No. 2, pp. 255-261
(1992), A. H. Rosee et al; Optics Letters, Vol. 18, No. 17, pp. 1471-1473
(1993); and U.S. Pat. No. 5,202,629). Further, optical integrated circuits
have been proposed in which optical waveguides are formed in a substrate
and different types of optical elements are disposed in the waveguides as
set out, for example, in Japanese Laid-open Patent Application No.
6360410. Optical parts or devices for optical communication have also been
proposed in which an optical fiber is embedded in grooves of a substrate,
and other grooves are separately formed in position by means of a rotating
blade saw in which optical elements are inserted, respectively (K.
Shiraishi et al; J. Lightwave Tech. Vol. 10, No. 12, pp. 1839-1842 (1992);
and Japanese Laid-open Patent Application No. 363606). Recently, a
line-assembled optical fiber sensor using a thin film polarizer has been
proposed for the purposes of miniaturization of the optical fiber sensor
and also of providing lenseless assemblings.
The measurement of an electric current passing through an electric wire is
shown in FIG. 9. In the figure, an optical current sensor including
conventional optical components is shown. An optical fiber magnetic field
sensor is disposed at a gap 2 of a horseshoe-type iron core 1 for use as
an optical fiber current sensor.
More particularly, the sensor includes a beam input side optical fiber 4,
an optical modulation unit M including a polarizer, a magnetooptical
crystal, an analyzer and a reflector (all not shown) and a beam output
side optical fiber 5 arranged as shown in the figure. A light beam from a
light source (not shown) is transmitted from the optical fiber 4 na the
optical modulation unit M to the optical fiber 6. At the center of the
horseshoe-type iron core 1, an electric wire 6 is passed through. The
passage of an electric current through the electric wire 6 permits a
magnetic field to be generated at the gap 2 of the iron core 1 in
proportion to the current to be measured. By this, the magnetooptical
crystal of the optical modulation unit M inserted into the gap 2 modulates
the light being transmitted in proportion to the magnitude of the electric
current.
FIG. 10 shows the relation between the generated magnetic field (Oe) at the
gap 2 and the gap distance in mm of the iron core 1 for an applied current
of 200 A. While the gap is changed from 6 to 35 mm, the intensity of the
magnetic field generated at the gap is measured. The horseshoe-type core
is made of a grain-oriented silicon steel. The electric wire is disposed
at the center of the core 1 and applied with a current of 200 A. From the
figure, it will be seen that the intensity of the magnetic field generated
in the gap becomes lower at a greater gap distance. In order to set
existing optical sensors in position, the gap distance of the steel core
should be approximately 20 mm or above. If this distance can be shortened
to about 10 mm, the current detection sensitivity increases to about two
times greater than that of the case using the distance of 20 mm. Likewise,
if it is possible to shorten the distance to about 5 mm, one is enabled to
improve the sensitivity by about 4 times greater.
FIG. 11 typically shows an arrangement of a prior art optical fiber
magnetic field sensor of the type being set in the gap 2 of the iron core
1 shown in FIG. 9. The sensor includes an optical modulation unit which
includes a polarizer 153, a magnetooptical element 154 made, for example,
of a garnet crystal, an analyzer 155 and a total reflection mirror 156
sequentially arranged in this order. The polarizer 153 is connected to an
input side optical fiber 163 via a collimating unit having a rod lens 152
and a ferrule 162. A holder 160 is to hold the lens 152 in position.
Likewise, the total reflection mirror 156 is connected to an output side
optical fiber 164 via a collimating unit U including a rod lens 167 and a
ferrule 162.
In this arrangement, a light beam from the optical fiber 163 is passed
through the collimating unit U, the optical modulation unit M, and the
collimating unit U' to the optical fiber 164. When a magnetic field is
applied to the magneto-optical element 154, the light beam being passed
therethrough is modulated proportionally to the intensity of the magnetic
field as is known in the art.
In practice, these optical parts or components including the optical fiber
163, lens 152, polarizer 153, magnetooptical element 154, analyzer 155,
mirror 156 and lens 157 are assembled and individually fixed by means of a
bonding agent while exactly adjusting the optical axes thereof.
This type of sensor has the following problems. A number of expensive
optical components and much time are required for assembling one optical
fiber sensor, resulting in a high fabrication cost. The polarizer 153,
analyzer 155 and total reflection mirror 156 have, respectively, a size of
about 5 mm square, thus impeding the fabrication of a sensor which is
small in size and thin. Although optical components having a size not
greater than 5 mm square may be commercially available, such parts are
very expensive and cannot withstand practical use.
The prior sensor will be improved in sensitivity if the core is made of a
material having high permeability and a high saturation magnetic flux
density or if the magnetooptical element is made of a crystal material
whose sensitivity is significantly improved or if the gap distance of the
horseshoe-type iron core is made narrower to prevent a leakage flux at the
gap portion thereby increasing the intensity of magnetic field thereat.
For the developments of such iron core materials and magneto-optical
materials, much cost and time are essential.
In contrast, the improvement of the sensitivity by reducing the gap
distance of the iron core or by hanging the shape of the gap portion might
be attained in an easier way. However, for the measurement of a magnetic
field or an electric current by utilizing light beams, it is essential to
insert the optical modulation unit including M the magneto-optical element
and other optical components into the gap as having been set out with
reference to FIG. 9. Thus, limitation is placed on the reduction in size
of the gap portion. The optical modulation unit is located in the optical
path of the optical fiber, and individual optical components are so
fragile that it is necessary to relatively loosely insert the optical
modulation unit into the gap portion. Accordingly, the gap distance of the
core depends solely on the sizes of the optical components.
In order to attain high sensitivity, the optical modulation unit should be
made so small in size that the horseshoe-type iron core can be designed to
have a narrow gap distance. Nevertheless, in view of the fact that the
elongated electric wire has to be set in position at the center of the
iron core, it is beneficial that the gap distance of the core be greater
than the diameter of the electric wire. To this end, it will be necessary
to arrange the core such that its gap distance is great enough to permit
easy insertion of the electric wire, but the gap is designed to be
magnetically narrow. In order to realize this arrangement and to improve
the sensitivity, Japanese Laid-open Patent Application No. 1308970 sets
out a problem to be solved. More particularly, such a problem includes how
to prevent leakage of a magnetic flux from the gap portion by forming a
path of the magnetic flux in the gap portion, thereby increasing the
intensity of a magnetic field to be applied to the optical modulation
unit.
Japanese Laid-open Patent Application No. 5-297027 sets forth a technique
for improving sensitivity without changing the distance in the gap of a
horseshoe-type iron core. More particularly, a magnetic piece and a
magnetic field detector element are integrally combined through an
insulating material. This integrally combined piece inserted into the gap
of the horseshoe-type iron core as a spacer for preventing a change of the
gap distance in the gap portion of the iron core. In the spacer
arrangement, the magnetic piece is in contact with one end portion of the
iron or steel core.
Japanese Laid-open Patent Application No. 3-170071 sets out a method
wherein a field-sensitive element is inserted into the gap of a
horseshoe-type iron core. Separately, iron core pieces which are made of a
material different from that of the horseshoe-type iron core are also
inserted into the gap. Thus, two types of iron core materials are
connected in series to improve current detection characteristics. The
purpose in this publication is to improve the hysteresis characteristics
of the iron core material, without positively reducing the gap distance of
the horseshoe-type iron core. In this method, the type of horseshoe-type
iron core and the type of iron core pieces being inserted into the gap
portion are properly selected depending on the magnitude of a current to
be measured.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an optical fiber current or
magnetic field sensor which includes an optical modulation unit and
optical fibers connected thereto without use of any lenses and/or
ferrules, so that the sensor can be made smaller in size than in prior art
counterparts.
It is another object of the invention to provide an optical fiber current
or magnetic field sensor which can realize a narrow gap distance in its
application to a horseshoe-type iron core.
It is a further object of the invention to provide an optical fiber sensor
which is small in size, simple in construction and low in cost.
It is a still further object of the invention to provide a method for
making an optical fiber sensor wherein a substrate is made of a
non-magnetic material portion and soft magnetic material portions provided
at opposite side of the first-mentioned portion and fixed to the
non-magnetic portion.
According to a broad aspect of the invention, there is provided an optical
fiber current or magnetic field sensor which comprises:
a substrate having a pair of elongated grooves separated from and in
parallel with each other and formed along the length of the substrate, and
a third groove intersected at right angles by the paired grooves,
respectively;
an optical fiber having a U-shaped portion which is inserted into and
forced in a groove pattern formed by the paired grooves and the third
groove; and
an optical modulation unit which is fixedly provided in a light path of the
optical fiber placed in the third groove, wherein the substrate is
composed of a composite substrate which composes a non-magnetic sheet
portion and soft magnetic portions fixed to the non-magnetic sheet portion
at opposite sides thereof, and the optical modulation unit is fixed to the
non-magnetic sheet portion.
Preferably, the U-shaped portion inserted in the groove pattern is made of
a bare optical fiber.
It will be noted that the term "bare optical fiber" is intended to mean an
optical fiber consisting of a core and a cladding layer but is free of any
jacket and a buffering layer although a surface treating agent may be
deposited on the cladding layers and used to permit intimate contact
between the cladding layer and the buffer layer.
In a preferred embodiment of the invention, the substrate is made of a
glass or ceramic portion and soft magnetic portions made of a soft
magnetic material and bonded to the glass or ceramic portion at opposite
sides thereof wherein the paired grooves are formed in the respective soft
magnetic portions and the optical modulation unit is disposed at the glass
or ceramic portion.
In accordance with another preferred embodiment of the invention, a soft
magnetic piece is mounted on at least one of the soft magnetic portions to
increase an area of a side of the sensor which is facing the end face of a
horseshoe-type iron core upon insertion of the sensor into a gap of the
iron core.
According to a further embodiment of the invention, there is also provided
a method for making an optical fiber current or magnetic field sensor
which comprises a substrate having a non-magnetic portion made of a
non-magnetic material and soft magnetic portions made of a soft magnetic
material and bonded at opposite sides of the non-magnetic portion, the
substrate having a pair of grooves separated from and in parallel with
each other and formed along the length of the substrate, and a third
groove intersected at right angles by the paired grooves, respectively; an
optical fiber having a U-shaped portion fixed in a groove pattern formed
by the paired grooves and the third groove; and an optical modulation unit
which is fixedly provided in a light path of the optical fiber placed in
the third groove, wherein the substrate is formed by bonding the soft
magnetic portions to the non-magnetic portion at opposite sides thereof by
means of a bonding agent. Alternatively, the soft magnetic portions may be
integrally combined with the nonmagnetic portion by applying an inorganic
bonding agent, such as a low melting glass bonding agent, between the soft
magnetic portions and the non-magnetic portion and firing the applied
bonding agent.
In the method, it is preferred that the non-magnetic portion is made of a
glass or ceramic material and has a thickness equal to or smaller than the
soft magnetic portions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG 1. is a schematic plan view of an optical fiber sensor only for
illustrating a fundamental concept of the invention;
FIG. 2 is a schematic perspective view of an optical fiber sensor according
to one embodiment of the invention;
FIG. 3 is a schematic perspective view of an optical fiber sensor according
to another embodiment of the invention;
FIG. 4 is a schematic perspective view of an optical fiber sensor according
to a further embodiment of the invention;
FIG. 5 is a side view of the sensor of FIG. 4 as viewed from the side of
the beam input and output optical fibers;
FIG. 6 is a schematic perspective view of an optical fiber sensor according
to a further embodiment of the invention as placed in a gap of a
horseshoe-shaped iron core;
FIG. 7 is a schematic perspective view of an optical fiber sensor according
to another embodiment of the invention;
FIG. 8 is a graph showing the relation between the modulation rate and the
electric current for one sensor in accordance with the present invention
and another sensor in accordance with the present invention;
FIG. 9 is a schematic perspective view showing application of an known
optical fiber sensor to detection of an electric current using a
horseshoe-type grain oriented silicon steel core;
FIG. 10 is a graph showing the relation between the magnetic field
generated in a gap of the core shown in FIG. 9 and the gap distance of the
grain oriented silicon steel core; and
FIG. 11 is a schematic plan view showing a known optical fiber sensor using
prior art optical components.
PREFERRED EMBODIMENTS OF THE INVENTION
Reference is now made to the accompanying drawings and particularly to FIG.
1. In FIGS. 1 to 8, like reference numerals indicate like parts or
members, respectively.
FIG. 1 shows a fundamental arrangement of an optical fiber sensor for
illustrating the general concept of the invention. It should be noted that
the fundamental arrangement shown in this figure is applicable to the
embodiments of the invention except that the substrate used differs from
those used in the embodiments of the invention as will be described
hereinafter. This type of sensor is set out, for example, in U.S. patent
application Ser. No. 08/497,353 or a corresponding European Patent
Application No. 9,511,0251.6.
The sensor is generally indicated by 6, which includes a substrate which is
entirely made of glass. The substrate 11 has a pair of relatively wide
grooves 15, 16 formed along the length thereof and a groove 17 which
intersects with the paired grooves 15, 16 at right angles, respectively,
thereby forming a groove pattern capable of receiving a U-shaped pattern
of an optical fiber as will be set out hereinafter. The paired grooves 15,
16 are kept away from and in parallel to each other as shown. The
substrate has further grooves 12, 13 and 14 formed in parallel to the
paired grooves 15, 16.
An optical fiber F has a U-shaped portion U and extensions 22, 23 from the
free ends of the U-shaped portion U. The U-shaped portion U, inverted as
viewed in the figure, of the optical fiber F is inserted into and fixed in
the groove pattern P formed by the paired grooves 15, 16 and the groove
17. The U-shaped portion U has two bent portions 22a and 23a,
respectively. The two bent portions are so formed as having a small radius
of curvature. The bottom portion of the U shape is linear as received in
the groove 17. The transmission loss of a light beam passing through the
optical fiber F varies depending on the curvature at the bent portions
22a, 23a. More particularly, a greater radius of curvature results in a
smaller loss and a smaller radius of curvature results in a greater loss.
If, however, the curvature is made great, it is inevitable that the width
of the sensor as a whole be much greater. This requires a corresponding
large gap distance of an horseshoe-type iron core to which the sensor is
applied.
The optical fiber F includes extensions 22, 23 and the bare or exposed
fiber portion U in the U form made of a core and a clad layer with or
without a surface treating agent on the clad layer. The extensions 22, 23
may be entirely composed of a jacketed optical fiber. Alternatively, a
bare optical fiber may be used for the respective extensions. The optical
fiber used may be any type of fiber known in the art and includes, for
example, multi-component optical fibers, plastic optical fibers, silica
optical fibers, and the like.
An optical modulation unit M is placed in the optical path of the 5 optical
fiber F along the groove 17 of the substrate 11.
The optical modulation unit M includes a polarizer 18, a magneto optical
element 19 and an analyzer 20. In this figure, the polarizer 18,
magnetooptical element 19 and analyzer 20 are, respectively, inserted into
the grooves 12, 13 and 14 from which the optical fiber has been removed.
By this, the optical modulation unit M is provided in the optical path.
The polarizer 18 and the analyzer 20 are, respectively, made of a thin
glass polarizing sheet, which is commercially available, for example from
Corning Inc. of United States under the designation of Polarcor. The
magnetooptical element 19 is made, for example of a garnet crystal. The
garnet single crystal used as the element 19 is formed, for example, by
epitaxially growing a Bi-substituted garnet film of the formula,
(BiYGdLa).sub.3 (FeGa).sub.5 O.sub.12 on a substrate made, for example, of
(GdCa).sub.3 (MgZrGa).sub.5 O.sub.12. As a matter of course, other types
of crystals such as (BiYGd).sub.3 Fe.sub.5 O.sub.12, (TbY).sub.3 Fe.sub.5
O.sub.12 and the like may also be used for this purpose.
The fabrication of the sensor of the type having set out hereinabove is
described.
First, the glass substrate is provided with a thickness of about 1 to 3 mm.
The substrate is formed with the paired grooves 15, 16 and the groove 17
according to any known procedures including the use of a rotating blade
saw, sand blasting, molding techniques and the like. It is preferred in
view of the grooving accuracy, processing speed and worldling properties
to use a rotating blade saw. The use of the rotating blade saw is very
advantageous from the standpoint of mass productivity.
Separately, an optical fiber is provided with bare or exposed portion by
removing a jacket therefrom. Then, the optical fiber is bent at a desired
bending angle by use of a blower with an electric resistance heater.
Preferably, the bent portions 22a, 23a are formed so as to have a
predetermined bending radius in view of the optical loss and the size of
the sensor. The thus shaped optical fiber E is fixed to a groove pattern
found by the grooves 15, 16 and 17 by using a bonding agent such as a
thermosetting epoxy resin. It should be noted that at least the grooves
15, 16 are formed wide enough to be sufficient to accommodate the bent
portions 22a, 23a therein.
Thereafter, the grooves 12, 13 and 14 are found, for example, by use of a
rotating blade saw, to interact at right angles with the groove 17. During
the formation, the bonded optical fiber is scraped off at the grooves 12,
13 and 14. The grooves 12, 13 and 14 should have, respectively, widths
which are slightly greater than the widths of the polarizer 18,
magnetooptical element 19 and analyzer 20 to be inserted thereinto.
In this arrangement, when an electric current is passed through an electric
wire in a manner shown in FIG. 9, the magnetic field is generated in the
direction shown by reference numeral 100.
If the bare optical fiber in the U form has a size greater than the grooves
15, 16 and 17, the fiber is not inserted thereinto. On the other hand,
when the optical fiber of the U-shaped portion is much smaller in size
than the grooves 15, 16 and 17, a greater amount of bonding agent becomes
necessary. This is not advantageous in that after insertion and bonding of
the optical modulation unit, the optical axes of individual optical
elements may be varied owing to a change in temperature. In general, if
the bare optical fiber has a diameter of 230 the groove 17 is formed using
a blade having a width of approximately 250 .mu.m. It will be noted that
the groove width formed by means of the rotating blade saw usually becomes
slightly greater than a thickness of the blade used. The grooves 15, 16
are found as having a width sufficient to receive the bent portions 22a,
23a, and the width is generally in the range of 0.5 to 3 mm.
Subsequently, the individual optical elements 18, 19 and 20 are,
respectively, inserted into and fixed in the grooves 12, 13 and 14 thereby
forming the magnetooptical modulation unit N. The grooves 12, 13 and 14
are formed as having, respectively, a width, for example, of 0.2 to 0.5
mm. This is because the thicknesses of the thin glass polarizer 18 and
analyzer 20 are, respectively, in the range of from 0.2 to 0.5 mm and the
garnet crystal used as the magnetooptical element 19 is, for example, one
obtained by liquid phase epitaxy and has a thickness of about 0.5 mm.
In this arrangement, the sensor can be made smaller in size since lenses
and/or ferrules as used in the sensor of FIG. 11 are unnecessary. In
addition, the optical elements or parts are inserted into the grooves and
fixed, so that it is unnecessary to adjust the optical axes of the
elements. A misalignment in optical axes of the elements can be caused in
conventional sensors by inclination of the elements, and adjustment of the
elements takes a very long time. In the sensor according to the present
invention, the problem on the adjustment is completely overcome.
The size in cross-section of the optical elements for transmitting a beam
should be slightly greater than the diameter of the optical fiber, and is
generally in the range of 0.5 to 2.0 mm. Thus, the miniaturization of the
sensor can be realized.
Nevertheless, if the substrate which is entirely made of glass or other
non-magnetic material has a width of 20 mm, it becomes necessary to make a
gap distance of a horseshoe-type iron core at approximately 26 mm while
taking into account a margin for module casing. In this case, the region
where the optical modulation unit including the polarizer, magnetooptical
element and analyzer is set has a width of about 10 mm, with the balance
being for the regions occupied by the bent portions of the optical fiber.
This is not advantageous from the standpoint of the sensor sensitivity as
previously discussed with reference to FIG. 10.
FIG. 2 shows an improved optical fiber sensor according to one embodiment
of the invention. In the embodiment, a substrate 110 is different from the
substrate 11 of FIG. 1 and is a composite substrate. The composite
substrate 110 is made of two types of materials. More particularly, the
composite substrate 110 includes a non-magnetic portion 111 and two soft
magnetic portions 112, 112' made of a soft magnetic material such as a
ferrite and bonded to the non-magnetic portion 111 at opposite sides
thereof as shown in FIG. 2. The non-magnetic portion has three
longitudinal grooves 12, 13, 14 extending along the length thereof. The
two portions 112, 112', respectively, have longitudinal grooves 15, 16
which are wider than the grooves 12, 13 and 14. The transverse groove 17
is formed to interact with the grooves 15, 16 at substantially right
angles.
Owing to the presence of the soft magnetic portions 112, 112' at opposite
sides of the composite substrate 110, the effective gap distance in a
horseshoe-type iron core can be reduced as previously discussed herein.
The non-magnetic materials for the portion 111 include, for example, glass
and ceramics made, for example, of SiO.sub.2, Al.sub.2 O.sub.3 and the
like, and synthetic resins such as glass-epoxy materials, phenolic resins
and the like. In view of the ease in processability and the processing
accuracy, glass or ceramic material is preferred.
The optical modulation unit M is disposed in the non-magnetic portion 111.
More particularly, at the intersections of the longitudinal grooves 12, 13
and 14 and the transverse groove 17. The polarizer 18 is inserted into the
groove 12, the magnetooptical element 19 is inserted into the groove 13,
and the analyzer 20 is inserted into the groove 14. These optical elements
18, 19 and 20 are forced to the respective grooves by means of a bonding
agent.
The sensor of this embodiment can be fabricated substantially in the same
manner as set out with reference to FIG. 1 except that the composite
substrate 110 is made in the following manner.
The non-magnetic sheet portion 111 is bonded with the soft magnetic
portions 112, 112' at opposite sides thereof by means of an organic
bonding agent such as an epoxy resin. Alternatively, an inorganic bonding
agent such as low melting glass may be used for the bonding, followed by
firing at a temperature ranging from 400.degree. to 650.degree. C. to
integrally bond the non-magnetic and magnetic portions together.
The composite substrate 110 generally has a thickness sufficient to fixedly
hold the U-shaped bare optical fiber U in the grooves 15, 16 and 17. In
view of the ease in handling and the yield which may be reduced by
breakage, the thickness of the substrate is preferably 1 mm or over.
For the efficient fabrication of the composite substrate 110, it is
convenient to bond an elongated non-magnetic sheet 111 and elongated soft
magnetic portions 112, 112' together in a manner as having set out
hereinabove to obtain an elongated composite material. This elongated
material is cut into pieces, each serving as the composite substrate 110
with a desired length. In this case, good productivity is attained when
the longitudinal grooves 15, 16 and the transverse grooves 17 have been
formed in the elongated composite material prior to the cutting. Needless
to say, it is not always necessary to bond the non-magnetic portion 111
and the soft magnetic portions 112, 112' so as to be exactly level at end
faces thereof In other words, these portions may be bonded so as to be
slightly stepped with each other.
The soft magnetic materials used for the portions or sheets 112, 112' may
be ferrites such as Mn--Zn ferrite, Ni--Zn ferrite and the like, or
silicon steels. Of these, the ferrites are preferred although this will
depend on the type of horseshoe-type core material.
When the sensor is inserted into a gap of a horseshoe-type iron core, the
effective gap distance can be reduced by the presence of the soft magnetic
portions 112, 112'. In addition, the bent portions 22a and 23a of the
optical fiber F are, respectively, disposed in the grooves 15 and 16 of
the soft magnetic portions 112, 112' . Accordingly, the bent portions 22a,
24a may be bent at a greater radius of curvature, thereby reducing beam
transmission loss without any increase in the effective gap distance. This
is advantageous in that even if the horseshoe-type iron core is designed
to have a large gap distance to sufficiently check an electric current of
an electric wire having a relatively great diameter, the effective gap
distance becomes far smaller than the actual designed gap distance.
Accordingly, this optical fiber sensor exhibits higher sensitivity than in
the case of a sensor using a substrate entirely made of glass or the like
non-magnetic materials. The sensor is especially effective in checking an
electric current passing through or a magnetic field generated around
relatively thick electric wires.
Preferably, the radius of curvature at the bent portions 22a, 23a is in the
range of 0.3 to 5 mm. If the bending radius of curvature is smaller than
0.3 mm, the optical loss exceeds 5 dB. In addition, it is difficult to
consistently bend the bare optical fiber portion at a smaller radius.
The optical fiber sensor using the composite substrate is negatively
influenced by only a reduced external magnetic field, and thus, has high
sensitivity.
FIG. 3 shows another embodiment of the invention. This embodiment differs
from the embodiment shown in FIG. 2 in that a composite substrate 210
includes a non-magnetic portion 111 bonded with soft magnetic portions
112, 112' at opposite sides, like the 20 composite substrate 110 of the
first embodiment, but the non-magnetic portion 111 is so small in width as
to form only one groove for insertion of the garnet or other
magnetooptical crystal element 19 alone.
The polarizer 18 and the analyzer are, respectively, inserted into and
fixed in the grooves 12, 14 formed in the soft magnetic portions 112, 112'
provided at opposite sides of the non-magnetic portion 111.
The non-magnetic portion 111 is of sufficient width so as to be slightly
wider than a thickness of the magnetooptical element 19.
A number of magnetooptical materials are known in the art, of which a
garnet crystal formed by liquid phase epitaxy as described hereinbefore is
the thinnest and exhibits the highest sensitivity. The garnet crystal
including a substrate for the epitaxy has a thickness as small as about
0.5 mm. Taking into account the thickness of the garnet crystal 19 and a
processing margin for the longitudinal groove 13, it is possible that the
non-magnetic portion 111 sandwiched between the soft magnetic portions
112, 112' is set at a thickness of about 2 mm. This means that the
distance between the soft magnetic portions 112, 112' can be brought close
to about 2 mm.
When the sensor of this embodiment is inserted into the gap of a
horseshoe-type iron core, the intensity of a magnetic field generated in
the gap of the iron core can be detected to a much greater extent than in
prior art counterparts, with enhanced prevention of leakage of the
magnetic flux. This leads to a small flux leakage loss at the gap of the
iron core, with the result that it is possible to detect, a magnetic field
from a very weak current passing through an electric wire placed
substantially at the center of the iron core. This arrangement ensures a
gap distance which is smaller than that shown in FIG. 2.
In the foregoing embodiments, if the composite substrates 111 are too thin,
a magnetic field generated at the gap of a horseshoe-type iron core may
not be satisfactorily detected due to the leakage thereof. In order to
increase the intensity of the magnetic field to be detected, it is
preferred to use soft magnetic portions which are thick enough to prevent
the leakage of the flux. In this case, the soft magnetic portions and the
non-magnetic portion may differ in thickness provided that the soft
magnetic portions are thicker than the non-magnetic portion.
This is particularly described with reference to FIGS. 4 to 6 to illustrate
the use of thick soft magnetic portions.
FIG. 4 shows an optical fiber sensor according to a further embodiment of
the invention wherein a composite substrate 310 includes a non-magnetic
portion 111 and soft magnetic portions 112, 112' bonded to the
non-magnetic portion 111 at opposite sides thereof in a manner similar to
the foregoing embodiments. In this embodiment, the longitudinal grooves
12, 13 and 14 are formed in the non-magnetic portion 111 although only one
groove 13 may be formed in the sheet 111 as is particularly shown in FIG.
3.
The soft magnetic portions 112, 112' are each thicker than the nonmagnetic
portion 111 sandwiched therebetween. In order to prevent the flux leakage
to a satisfactory extent, the soft magnetic portions 112, 112' should
preferably have a thickness ranging from 10 to 30 mm, which may vary
depending on the area of opposing faces of a horseshoe-type iron core
between which the gap is established.
For fabrication of this type of composite substrate 310, the soft magnetic
portions 112, 112' are bonded to the non-magnetic portion 111 at opposite
sides thereof. The grooves 15, 16 are, respectively, formed in the sheets
112, 112'. Thereafter, the transverse groove 17 is formed along the width
of the composite substrate 310 at right angles to the grooves 15, 16 as
shown. The U-shaped portion of the optical fiber F is set in position in
the grooves 15, 16 and 17 and bonded to the grooves in the same manner as
in the foregoing embodiments. Subsequently, the non-magnetic portion 111
is grooved as indicated to form grooves 12, 13 and 14, each to a depth
where the optical fiber has been set in position. More particularly,
although the depth depends on the diameter of the bare optical fiber U, it
should be greater than the diameter of the bare optical fiber 11, thereby
completely cutting and removing the optical fiber at the respective
grooves.
FIG. 5 shows the substrate 310 after insertion of the optical modulation
unit as viewed from beam output and input sides.
In the figure, it will be seen that the central non-magnetic portion 111 is
integrally bonded with the soft magnetic portions 112, 112' at opposite
sides thereof to provide the composite substrate 310. The polarizer 18,
magnetooptical element 19 and analyzer 20 are inserted into the grooves
12, 13 and 14, respectively. Optical fibers 24 are left between adjacent
optical elements 18, 19 and 20, thereby forming a light path. In FIG. 5,
the bare optical fiber 22 at the beam input side and the bare optical
fiber 23 at the beam output side are, respectively, shown as not jacketed
in the grooves 15, 16. In this connection, the optical fibers 22, 23 may
be fixed in the groove entrance regions of the grooves 22, 23 as jacketed.
This is advantageous in that the optical fiber is unlikely to break at the
groove entrances.
In FIG. 5, the polarizer 18, magnetooptical crystal element 19 and analyzer
20 are, respectively, disposed in the grooves 12, 13 and 14 so as to be
higher than the depths of the individual grooves. This is for the
following reason. When these optical elements are inserted into the
respective grooves, they have to be carefully set so that the setting
directions thereof are not in error. Nevertheless, the optical elements
are very fine in size. If an error in the direction occurs, elements set
higher than the grooves can be readily detached. Of course, the grooves
12, 13 and 14 may be so formed as to have a depth greater than the heights
of individual optical elements.
FIG. 6 shows another embodiment of an optical fiber sensor which has the
substrate 310 as shown in FIGS. 4 and 5. In this embodiment, another
composite substrate 310' is mounted on the substrate 310 having the
optical fiber F and the optical modulation unit M therein. The substrate
310' has neither any optical fiber F nor optical modulation unit M but has
a non-magnetic portion 111 and soft magnetic portions 112, 112' bonded to
the non-magnetic portion 111 at opposite sides thereof, like the substrate
310. The soft magnetic portions 112, 112' of the composite substrate 310'
are, respectively, bonded to the portions 112, 112' of the composite
substrate 310 in a manner as shown hereinbefore. The total thickness of
the soft magnetic portions of the composite substrates 310, 310' is so
controlled as to substantially coincide with the thickness of a
horseshoe-type iron core 25 at a gap. Although not shown in FIG. 6, the
width of the sensor having the combination of the substrates 310, 310'
should preferably be substantially coincident with that of the iron core
25. In this arrangement, the gap G of the iron core 25 is substantially
entirely filled with the soft magnetic portions 112, 112'. Although the
superposition of the substrate 310' having the non-magnetic portion 111 on
the substrate 310 has been set out above, soft magnetic pieces alone may
be mounted on the corresponding soft magnetic portion portions 112, 112'
of the composite substrate 310, with similar results.
By the arrangement set out above, leakage of the magnetic field generated
in the gap G of the iron core 25 can be prevented and the magnetic field
may be concentrated at the optical modulation unit M. In FIG. 6, although
a bonding agent 26 is shown as being built up, the substrate 310' can be
superposed without any influence of the built-up agent 26.
FIG. 7 shows yet another embodiment of an optical fiber sensor according to
the invention. In this embodiment, a composite substrate 410 is composed
of a central non-magnetic portion 111 and soft magnetic portions 112, 112'
bonded with the non-magnetic portion 111 at opposite sides thereof.
In this case, a polarizer 18, an optical modulation element 19 made of a
garnet crystal, and an analyzer 20 sequentially arranged in this order as
in the forgoing embodiments are integrally bonded together by means of
such bonding agents as set out hereinbefore, thereby forming a integrally
combined optical modulation unit M. This integrally combined unit M is
inserted into and fixed in a groove 28.
Most commercially available glass analyzed and polarized have,
respectively, 0.2 mm in thickness. When these non-magnetic portions are
bonded together for use as the polarizer 18 and the analyzer 20, the total
thickness becomes 0.4 mm. The optical modulation element 19 such as a
garnet crystal has to be sandwiched between the polarizer 18 and the
analyzer 20, such that the integrally combined unit M has a thickness of
approximately 1 mm. When a garnet substrate on which a garnet crystal 19
is epitaxially grown is polished, the total thickness of the optical
modulation unit M can be further reduced.
In this embodiment, only one longitudinal groove 28 which is slightly wider
than the thickness of the integrally combined modulation unit M is formed
in the central non-magnetic portion 111. Accordingly, the non-magnetic
portion 111 can be designed to have a width of approximately 2 mm. Thus,
the gap distance in the horseshoe-type iron core can actually be produced
by the bonding of the soft magnetic portions 112, 112' at opposite sides
of the non-magnetic portion 111. The formation of only one groove 28 in
the non-magnetic portion 111 is more efficient and simpler. In addition,
the optical modulation unit M can be readily obtained by bonding elongated
optical parts together and cutting them into pieces with a desired size.
FIG. 8 shows the relation between the modulation ratio and the electric
current passing through an electric wire in a horseshoe-type iron core of
the type shown in FIG. 9 for the sensor of the invention using ferrite
sheets and a sensor for comparison as shown in FIG. 1 which makes use of a
glass substrate alone. The sensor comprising the ferrite sheets is of the
type shown in FIG. 2.
The gap distance in the horseshoe-type iron core is set at 26 cm, and an
electric wire is provided substantially at the center of the core.
The sensor which has a width of a glass substrate of 20 cm, is inserted
into the gap of the iron core, followed by the passage of different
electric currents to detect a modulation signal.
Likewise, the sensor of FIG. 2, which has a width of a glass portion of 8
mm and a width of each ferrite portion of 6 cm, with a total thickness
corresponding to the width of the glass substrate sensor, is inserted into
the gap and subjected to measurement of the modulation signal.
The results are shown in FIG. 8, wherein the line obtained by plotting
solid circles is for the sensor of FIG. 2 and the line obtained by
plotting circles is for the glass substrate sensor of FIG. 1 for
comparison.
With the sensor for comparison, the modulation rate is about 18% at an
electric current of 200 A. On the other hand, with the sensor of FIG. 2
which has a substrate made of the glass portion and ferrite portions
bonded at opposite sides thereof, the modulation ratio is about 33% at 200
A. Thus, the sensitivity of the sensor of FIG. 2 is about two times that
of the comparison sensor of FIG. 1.
Moreover, when the above procedure is repeated using an optical fiber
sensor of the type shown in FIG. 7, which has a glass portion of 2 mm in
width and each ferrite portion of 9 mm in width, the modulation ratio
reaches 56% at an electric currant of 200 A. The sensitivity of this
sensor is three times greater than that of the comparative sensor.
In the fabrication of the sensors according to the foregoing 10 embodiments
of the invention, it is efficient from the standpoint of productivity to
initially provide an, elongated non-magnetic portion and elongated soft
magnetic sheets. Then, these sheets are bonded together and grooved in a
manner as having been set out hereinbefore, followed by cutting the sheets
into unit pieces having a desired length. Accordingly, a number of
substrates can be efficiently made. Subsequently, a shaped optical fiber
is set and fixed in the grooves of the thus made substrate unit and an
optical modulation unit is also set and fixed in a light path of the
optical fiber.
Alternatively, elongated non-magnetic and soft magnetic sheets may be
bonded together with a length corresponding to that necessary for two
sensors, followed by forming two grooves along the length thereof and two
transverse grooves in such a way that two shaped optical fibers are set
and forced in the respective groove patterns each made of the two
lengthwise grooves and one transverse groove. In order to set an optical
modulation unit in a light path of the optical fiber, at least one groove
is formed in parallel to the lengthwise grooves and set with the optical
modulation unit in position. Thus, two sensors can be fabricated at one
time, with good productivity.
In the foregoing embodiments, the ferrites are mentioned as the soft
magnetic material. It is known that the magnetic permeability of ferntes
vary depending on the ambient temperature. The magnetic permeability also
varies, more or less, depending on the type, shape and dimension of
ferrite. In the practice of the invention, it may be possible to use an
optical modulation element made of a garnet crystal whose temperature
characteristics of sensitivity is inconsistent with the temperature
dependence of permeability of the permeability of the ferrite used.
The magnetic permeability of ferrites increases at higher temperatures. To
compensate for the increase of the magnetic permeability due to the
influence of a rise in temperature, the garnet crystal used should
conveniently be made, for example, of Bi.sub.1.lGd.sub.0.85 Y.sub.1.05
Fe.sub.4.5 Ga.sub.0.05 O.sub.12 which has reduced sensitivity when the
temperature increases. In this way, the resultant optical fiber sensor
becomes stable with respect to the change of the ambient temperature.
In the practice of the invention, it is preferred to use a ferrite both for
a soft magnetic portion of a composite substrate and for a horseshoetype
iron core. However, another type of soft magnetic material be used as the
soft magnetic portion without causing any lowering of sensitivity. For
instance, silicon steels may be used for this purpose. In view of the cost
and performance of optical fiber sensors, the iron core may be made of a
soft magnetic material other than ferrites, e.g. silicon steels. When
different types of soft magnetic materials are used for the composite
substrate and the iron core and a garnet crystal is used to compensate for
a variation in magnetic permeability due to the temperature, care should
be taken as to which of the material, used for the composite substrate or
the core is more influenced by the variation.
Moreover, according to the invention, a non-magnetic portion and soft
magnetic portions are bonded together by means of a bonding agent. If the
resultant composite substrate is not satisfactory with respect to the
mechanical strength thereof, a reinforcing sheet made, for example, of a
synthetic resin may be attached to a side of the optical fiber sensor
which is opposite to a side on which the optical modulation unit has been
formed.
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